When colors of a first color-image output device are reproduced by a second color-image output device whose gamut is different from the gamut of the first color-image output device, there is used a method to convert CMYK values representing colors of the first color-image output device into color system values (L*a*b* values) and to convert the color system values into CMYK values representing colors of the second color-image output device. At that time, because color system values usually cover a wider region than the gamut (a color-reproducible region) of a color-image output device, there is preformed, in creating a lookup table for converting color system values into CMYK values, processing to adjust the color system values of out-gamut colors, which are located outside the gamut of a color-image output device, so as to move the out-gamut colors within the gamut (which is referred as a gamut mapping).
As for the gamut mapping, for example, JP-A No. 2003-78773 discloses a color-adjusting method of performing a color conversion of image data so as to obtain an output which is consistent to colors of a first color-image output device by using a second color-image output device. In a step of creating a lookup table in the method, the following operation is performed for combinations of color system values which correspond to colors located outside a color-reproducible region reproduced by the color-image output device using basic color values consisting of three or four color components. In the condition that the lightness value of a color is greater than the lightness value of the point of maximum chroma of the color-reproducible region at the hue of the color, the color is moved inside the color-reproducible region in the direction to reduce its chroma value and its lightness value. In the condition that the lightness value of a color is smaller than the lightness value of the point of maximum chroma, the color is moved inside the color-reproducible region in the direction to reduce its chroma value and increase its lightness value. Thereby, values of basic colors consisting of four or three color components are calculated for a combination of color system values of each color outside the color-reproducible region.
Further, printing on thin paper by using a color-image output device, sometimes causes a situation that the quantity of color material to be used is requested to be reduced in order to prevent color material on the reverse side from showing through the paper. In order to save a consumption of resources for printing, it has been performed to employ a color-material-saving function to use the reduced quantity of color material. When the quantity of color material used is saved as described above, the gamut which is reproducible by a color-image output device becomes narrow, which makes a phenomenon that the color-reproducing accuracy, especially the color-reproducing accuracy in the low-lightness area, is deteriorated.
To solve the problem, for example, JP-A No. 2009-212638 discloses the method to be used under the condition that a first color signal representing colors described by a certain color space is converted into a second color signal representing colors described by another color space handled by an image forming device. Under the condition, a limiting value of the total amount of the second color signal, which is defined for each of colors represented by the first color signal, is calculated in the following manner. The limiting value is increased when chroma of a color represented by the first color signal is high, and is reduced when the chroma of a color represented by the first color signal is low. Further, the limiting value is increased when the lightness of a color represented by the first color signal is high and is reduced when the lightness of a color represented by the first color signal is low.
However, in the method of JP-A No. 2003-78773, a gamut mapping is performed for each color in the region where lightness values are lower than the lightness value of the point of maximum chroma (referred as a low-lightness region) so as to reduce its chroma value and increase its lightness value without exception, regardless its chroma value. Therefore, when the gamut of the second color-image output device is much narrower than the gamut of the first color-image output device, lightness values greatly change in the middle-chroma and high-chroma regions in the low-lightness region, which makes a problem that the color-reproducible accuracy and gradation are deteriorated.
The method of JP-A 2009-212638 allows the color-reproducing accuracy to be maintained in a certain extent. However, the limiting value of the total amount of color signal of CMYK becomes large in a high-chroma region. Therefore, in an image wherein a color region of second colors is great, the limiting value becomes large in total, which makes a problem that it goes against the purpose to reduce the limiting value. The present invention seeks to solve these problems.